Most people just accept that a car battery works. However, to a chemist it is a perplexing mystery because the prime ingredient (lead oxide) should be an insulator. Chemists have solved the 150 year-old mystery of what gives the lead-acid car battery its unique ability to deliver a surge of current. Lead-acid batteries are able to deliver the very large currents needed to start a car engine because of the exceptionally high electrical conductivity of the battery anode material, lead dioxide. However, even though this type of battery was invented in 1859, up until now the fundamental reason for the high conductivity of lead dioxide has eluded scientists.

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Lead-acid batteries are made up of plates of lead and separate plates of lead dioxide, which are submerged into an electrolyte solution of about 35% sulfuric acid and 65% water. This causes a chemical reaction that releases electrons, allowing them to flow through conductors to produce electricity. As the battery discharges, the acid of the electrolyte reacts with the materials of the plates, changing their surface to lead sulfate. When the battery is recharged, the chemical reaction is reversed: the lead sulfate reforms into lead oxide and lead. With the plates restored to their original condition, the process may now be repeated.

A team of researchers from Oxford University, the University of Bath, Trinity College Dublin, and the ISIS neutron spallation source, have explained for the first time the fundamental reason for the high conductivity of lead dioxide.

A report of the research appears in this week’s Physical Review Letters.

"The unique ability of lead acid batteries to deliver surge currents in excess of 100 amps to turn over a starter motor in an automobile depends critically on the fact that the lead dioxide which stores the chemical energy in the battery anode has a very high electrical conductivity, thus allowing large current to be drawn on demand," said Professor Russ Egdell of Oxford University’s Department of Chemistry, an author of the paper.

"However the origin of conductivity in lead oxide has remained a matter of controversy. Other oxides with the same structure, such as titanium dioxide, are electrical insulators."

Through a combination of computational chemistry and neutron diffraction, the team has demonstrated that lead dioxide is intrinsically an insulator with a small electronic band gap, but invariably becomes electron rich due to the loss of oxygen from the lattice, causing the material to be transformed from an insulator into a metallic conductor.